Nuclear Fission Nuclear Fission is when a large nucleus splits into two smaller nuclei, releasing neutrons and energy –Heavier nuclei tend to be more unstable Energy a small amount of mass changes to energy E = mc 2 Matter and energy are interchanged in a nuclear reaction (the mass of the products is slightly less than the mass of the reactant) All nuclear power generation is accomplished through nuclear fission. n 1 0

Nuclear reactions vs Chemical reactions Chemical reactions: –mass is conserved, and energy changes are relatively small. –There are no changes to the nuclei in chemical reactions. Nuclear reactions: –the actual nucleus of atoms changes. –Protons, neutrons, electrons, and/or gamma rays can be lost or gained. –Small changes of mass = huge changes in energy –The nuclear fission of 1 g of uranium-235 produces the same amount of energy as burning 2 tonnes of coal!

Induced Nuclear Reactions In addition to natural radioactive decay, scientists can also create nuclear reactions by smashing nuclei with alpha, beta and gamma radiation or other subatomic particles.

Nuclear Fission of U-235 It is much easier to crash a neutral neutron than a positive proton into a nucleus to release energy. –Most nuclear fission reactors and weapons use this principle. –A neutron crashes into an atom of stable U-235 to create unstable U-236, which then undergoes radioactive decay. –After several steps, atoms of krypton and barium are formed, along with the release of three neutrons and huge quantities of energy.

Chain Reactions Once the nuclear fission reaction has started, it can keep going. –The neutrons released trigger more reactions on other U-235 atoms. –This chain reaction can quickly get out of control. –materials that absorb neutrons help to control the chain reaction. eg: Cadmium Rods –Nuclear bombs are uncontrolled reactions

CANDU Reactors CANadian Deuterium Uranium reactor –safe and easy to shut down in an emergency. –produces a lot of heat to generate steam to drive a turbine that produces electricity. Hazardous wastes –Some waste products, like fuel rods, can be re-used. –Some products are very radioactive, however, and must be stored away from living things. –Most of this waste is buried underground or stored in concrete. –It will take 20 half-lives (thousands of years) before the material is safe.

Nuclear Fusion Nuclear Fusion is when two small nuclei join together into one larger nucleus, releasing large amounts of energy Energy Nuclear Fusion produces much more energy for a given mass of fuel than Nuclear Fission. e.g.:  The Sun and other stars  Hydrogen Bomb  the basic source of energy in the universe (how all elements are formed)

Nuclear Fusion continued As an energy source it is clean and plentiful, however it is very hard to control as it needs very high temperatures (100,000,000°C) Scientists cannot yet find a safe, manageable method to harness the energy of nuclear fusion. –So-called “cold fusion” would occur at temperatures and pressures that could be controlled.

Page 315 “Reading Check” Q.1-5 Page 317 “Practice Problems” Q.1 Workbook Pages